1. Field of the Invention
The present invention relates to a path status monitoring method and device, and in particular to a method and device for monitoring signal reception faults which occur in parallel physical paths.
2. Description of the Related Art
The above-mentioned path status monitoring technology is applied to an EoS (Ethernet Over SONET) where Ethernet (registered trademark) data is transmitted to a SONET network, or the like.
Hereinafter, general transmission examples [1] and [2] of the EoS to which the path status monitoring technology is not applied will be firstly described referring to FIGS. 24A-25. Then, a transmission example of the EoS to which a prior art path status monitoring technology is applied will be described referring to FIGS. 26A-32.
General Transmission Example [1] of EoS (CCAT): FIGS. 24A-24C
SONET employs a contiguous concatenation (CCAT) container (hereinafter, referred to as STS line or merely path), prescribing an STS-1 as a basis where an SPE (Synchronous Payload Envelope) is “49.536 Mbps”, an STS-3c (SPE=“149.76 Mbps”), an STS-12c (SPE=“599.04 Mbps”), an STS-48c (SPE=“2396.16 Mbps”), an STS-192c (SPE=“9584.64 Mbps”), and the like, so that a transmission in a broad bandwidth is realized.
For example, in a case where a transmission rate of the Ethernet data DT is “10 Mbps” as shown in FIG. 24A, the data DT is transmitted to the SONET network by using an STS-1 line whose transmission rate is higher than that of the data DT. Similarly, an STS-3c line is used in a case where the transmission rate of the Ethernet data DT is “100 Mbps” as shown in FIG. 24B, and an STS-48c line is used in a case where the transmission rate of the Ethernet data DT is “1000 Mbps” as shown in FIG. 24C.
However, in these cases, the transmission rate of the STS line is considerably higher than the actual transmission rate. Accordingly, it has been disadvantageous that the STS line can not be efficiently used.
As a technology to address this problem, a VCAT (Virtual Concatenation) has been already proposed (see e.g. non-patent document 1), which will be described hereinbelow.
General Transmission Example [2] of EoS (VCAT): FIG. 25
In the VCAT, the Ethernet data DT are transmitted in parallel by using a plurality of STS lines respectively having transmission rates lower than those of the Ethernet data DT.
Namely, as shown in FIG. 25 for example, a transmission device 1_1 having received the Ethernet data DT through an Ethernet line EL1 from the precedent Ethernet network (not shown) divides the data DT into SONET frames FR to be transmitted to the SONET network through physical paths P0-Pn-1 which are formed of “n” pieces of STS lines. In FIG. 25, “STSx-nV” indicates that “n” pieces of STS lines having transmission rates lower than those of the Ethernet data DT are used. For example, in a case where the transmission rate of the Ethernet data DT is “1000 Mbps”, “24” pieces of STS-1 lines (STS1-24V) are used or “8” pieces of STS-3c lines (STS3c-8V) are used.
A transmission device 1_2 having received the SONET frames FR through the paths P0-Pn-1 from the SONET network assembles the original Ethernet data DT from the frames FR and transmits the Ethernet data DT assembled to the subsequent Ethernet network (not shown) through an Ethernet line EL2.
Thus, it is made possible to efficiently transmit the Ethernet data by combining in parallel and operating the STS lines whose transmission rates are lower.
However, when a band (frequency bandwidth) is enhanced or reduced in the VCAT (namely, when the number of operating paths is enhanced or reduced), there has been a problem that band settings of the transmission devices 1_1 and 1_2 have to be changed after all of the paths have been once made a communication disconnected state. Also, even when a reception fault of the SONET frames FR has occurred in a certain path on the transmission device 1_2 side, the transmission device 1_1 continues to transmit the frames FR through the path. Accordingly, there has been another problem that the Ethernet data DT assembled at the transmission device 1_2 results in abnormal data.
As a technology to address these problems, an LCAS (Link Capacity Adjustment Scheme) has been already proposed (see e.g. non-patent document 2), which will be described hereinbelow.
Transmission Example of EoS where Prior Art Path Status Monitoring Technology is Applied (LCAS): FIGS. 26A-32
In the LCAS, the enhancement or reduction of the band can be performed without causing the communication disconnected state and with minimizing the occurrence of the abnormal data as will be described in the following control examples (1)-(4):
LCAS Control Example (1) (Band Enhancement): FIGS. 26A, 26B, and 27
When the number of operating paths is added as shown in FIG. 26B in a state where communications are being performed with the paths P0-Pn-1 used as the operating paths between the transmission devices 1_1 and 1_2 as shown in FIG. 26A, the transmission device 1_1 firstly refers to a path status MST received from the transmission device 1_2 as shown by the solid lines in FIG. 27, and determines whether or not the reception fault has occurred in a path Pn desired to be switched over to the operating state on the transmission device 1_2 side.
In the path status MST, “OK” is set for the path where no reception fault has occurred and “FAULT” is set for the path where a reception fault has occurred.
When the above-mentioned determination results in that no reception fault has occurred in the path Pn, the transmission device 1_1 sets an additional operation request (“ADD”) for the path Pn in path use mode control information CTRL shown in FIG. 27 to be transmitted to the transmission device 1_2.
In addition to the above-mentioned “ADD”, contents which will be described hereinbelow can be set in the path use mode control information CTRL.    IDLE: The path Pn is in a non-operating state.    NORM: The transmission is normally performed in the path Pn.    DNU: An operation stop request for the path Pn.    EOS: The path Pn is the final path within a group.    FIXED: No control by the LCAS is performed.
The transmission device 1_2 transmits a response RS_Ack to the additional operation request “ADD” to the transmission device 1_1 as shown in FIG. 27 after having completed preparing the reception from the path Pn.
The transmission device 1_1 having received the response also transmits the SONET frames FR to the path Pn as shown by long and short dashed lines in FIG. 26B in addition to the paths P0-Pn-1.
Thus, the band can be enhanced without affecting at all the paths P0-Pn-1 except the path Pn to be switched over to the operating state.
LCAS Control Example (2) (Band Reduction): FIGS. 28A and 28B
When the number of operating paths is reduced as shown in FIG. 28B in a state where communications are being performed with the paths P0-Pn-1 used as the operating paths between the transmission devices 1_1 and 1_2 as shown in FIG. 28A, the transmission device 1_1 firstly stops the transmission of the SONET frames FR to the path Pn-1 desired to be switched over to the non-operating state. Concurrently, the transmission device 1_1 sets the above-mentioned operation stop request “DNU” for the path Pn-1 in the path use mode control information CTRL to be transmitted to the transmission device 1_2.
The transmission device 1_2 transmits the response RS_Ack to the operation stop request “DNU” to the transmission device 1_1 after having completed stopping the reception from the path Pn-1.
The transmission device 1_1 having received the response switches the path Pn-1 over to the non-operating state as shown by long and short dashed lines in FIG. 28B.
Thus, the band can be reduced without affecting at all the paths P0-Pn-2 except the path Pn-1 to be switched over to the non-operating state.
LCAS Control Example (3) (Upon Occurrence of Reception Fault): FIGS. 29A and 29B
As shown in FIG. 29A, when reception faults have occurred in e.g. the paths P1 and P2 among the paths P0-Pn-1 in operation in the transmission device 1_2, the transmission device 1_1 firstly detects that reception faults have occurred in the paths P1 and P2 based on the path status MST having been received from the transmission device 1_2, and stops transmitting the SONET frames FR to the paths P1 and P2. Concurrently, the transmission device 1_1 sets the operation stop request “DNU” for the paths P1 and P2 to the path use mode control information CTRL to be transmitted to the transmission device 1_2.
The transmission device 1_2 transmits the response RS_Ack to the operation stop request “DNU” to the transmission device 1_1 after having completed stopping the reception from the paths P1 and P2.
The transmission device 1_1 having received the response switches the paths P1 and P2 over to the non-operating state as shown by long and short dashed lines in FIG. 29B.
Thus, the abnormal data which occurs due to the reception faults of the paths P1 and P2 can be minimized.
LCAS Control Example (4) (Upon Recovery of Reception Fault): FIGS. 30A and 30B
As shown in FIG. 30A, when the reception faults of the paths P1 and P2 in the transmission device 1_2 shown in FIG. 29A have recovered, the transmission device 1_1 firstly detects from the path status MST having been received from the transmission device 1_2, that the reception faults of the paths P1 and P2 have recovered and sets the additional operation request “ADD” for the paths P1 and P2 in the path use mode control information CTRL to be transmitted to the transmission device 1_2.
The transmission device 1_2 transmits the response RS_Ack to the additional operation request “ADD” to the transmission device 1_1 after having completed preparing the reception from the paths P1 and P2.
The transmission device 1_1 having received the response switches the paths P1 and P2 over to the operating state again as shown in FIG. 30B, and transmits the SONET frames FR to the paths P1 and P2.
In the above-mentioned control examples (1)-(4), while the transmission devices 1_1 and 1_2 are respectively described as being on the receiving side of the path status MST and the transmitting side of the path status MST, the transmission and reception of the path status MST where the receiving side and transmitting side are reversed are similarly executed. Namely, the path status MST is monitored mutually between the transmission devices 1_1 and 1_2.
Hereinafter, a prior art example of the path status monitoring will be described in more detail, referring to FIGS. 31A, 31B, and 32.
Prior Art Example of Path Status Monitoring: FIGS. 31A, 31B, and 32
As shown in FIG. 31A, a transmission device 1S transmitting the path status MST cyclically generates the SONET frames FR which are serially connected over 32 frames and to which frame Nos. FN (“0”-“31”) are respectively assigned in a cycle Tc (=“64 ms”), and transmits the frames FR to paths P0-P2 with the same phases (transmission cycle per frame=“2 ms”).
In the specification of LCAS, it is prescribed that path statuses MST for an operation upper limit number (=“256”) are transmitted and received between the transmission devices 1S and 1R regardless of the number of actually existing paths. In each of the SONET frames FR, the path statuses MST for 8 paths are set according to the associated relationship between the frame Nos. FN and the path statuses MST as shown in FIG. 31B.
Namely, in the SONET frame FR whose frame No. FN is “0”, path statuses MST0-MST7 of the existing paths P0-P2 and non-existing paths P3-P7 are set. In the SONET frames FR whose frame Nos. FN are “1”, . . . , “30”, and “31”, path statuses MST8-15 of non-existing paths P8-P15, . . . , path statuses MST240-247 of non-existing paths P240-P247, and path statuses MST248-255 of non-existing paths P248-P255 are respectively set.
Also, the transmission device 1S sets “OK” or “FAULT” to the path statuses MST0-MST2 according to the statuses where the reception faults have occurred in the paths P0-P2 every time the SONET frame FR whose frame No. FN is “0” is generated, but constantly sets “FAULT” to the path statuses MST3-MST255 of the non-existing paths P3-P255.
Also, each of the SONET frames FR is multi-framed so that the path statuses MST for 8 paths may be stored in an H4 byte within a path overhead (POH) at one time.
FIG. 32 shows a format example of the above-mentioned H4 byte. In the H4 byte, bits 1-4 of each line are used as a data area and bits 5-8 of each line are used as a multi-frame indicator MFI.
The frame No. FN is set in the bits 1-4 of the lines where the multi-frame indicator MFI indicates “0000” and “0001” (namely, the “0th” and “1st” lines), and the path status MST is set as data for a total of 8 bits having “OK” (0) or “FAULT” (1) per one bit in the bits 1-4 of the lines where the multi-frame indicator MFI indicates “1000” and “1001” (namely, the “8th” and “9th” lines). Also, a path No. PN uniquely identifying the paths P0-P2 to which the SONET frames FR are transmitted is set in the bits 1-4 of the “14th” and “15th” lines.
The transmission device 1R (path status MST receiving side) having received the SONET frames FR through the paths P0-P2 extracts the path status MST and the frame No. FN from each of the SONET frames FR, and identifies which path's occurrence status of reception fault the path status MST extracted corresponds to, according to the associated relationship shown in FIG. 31B.
Thus, the transmission device 1R can recognize whether or not a reception fault has occurred in a desired path on the transmission device 1S, or whether or not the reception fault has recovered, thereby enabling the band described in the above-mentioned control examples (1)-(4) to be enhanced or reduced.
Also, the format of the H4 byte shown in FIG. 32 is commonly used when the path use mode control information CTRL shown in FIG. 27 and the response RS_Ack thereto are transmitted. The path use mode control information CTRL and the response RS_Ack are respectively set in the bits 1-4 of the “2nd” line and the bit 4 of the “10th” line.
Also, as other information, a group identifier GID upon dividing the paths into groups and an error detecting code CRC of the SONET frame FR can be respectively set in the bit 4 of the “3rd” line, and the bits 1-4 of the “6th” and “7th” lines.
It is to be noted that there has been proposed a reference example where shifts of phases between the frames caused by a difference between paths are adjusted on the receiving side of the SONET frame (see e.g. patent document 1).    [Non-patent document 1] ITU-T G.707    [Non-patent document 2] ITU-T G.7042/Y.1305    [Patent document 1] Japanese Patent Application Laid-open No. 2002-232380
In the above-mentioned prior art example, the enhancement or reduction of the band is performed based on the path statuses cyclically transmitted and received in the predetermined cycle. However, in a specific path noticed, the path status thereof can be monitored only per predetermined cycle. Accordingly, there has been a further problem that the reception fault or the recovery thereof which may occur during the cycle can not be detected until the next cycle, so that the enhancement or reduction of the band is delayed.
Particularly, the delay of reduction of the band upon the occurrence of reception faults may lead to an enormous occurrence of abnormal data. In the worst case, such a delay leads to a failure of the transmission device, a communication disconnection, and the like.